Method for fabricating a substrate and semiconductor structure

ABSTRACT

The invention relates to a method for fabricating a substrate, comprising the steps of providing a donor substrate with at least one free surface, performing an ion implantation at a predetermined depth of the donor substrate to form an in-depth predetermined splitting area inside the donor substrate, and is characterized in providing a layer of an adhesive, in particular an adhesive paste, over the at least one free surface of the donor substrate. The invention further relates to a semiconductor structure comprising a semiconductor layer, and a layer of a ceramic-based and/or a graphite-based and/or a metal-based adhesive provided on one main side of the semiconductor layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. §371 ofInternational Patent Application PCT/IB2012/002793, filed Dec. 21, 2012,designating the United States of America and published in English asInternational Patent Publication WO2013/102788 A1 on Jul. 11, 2013,which claims the benefit under Article 8 of the Patent CooperationTreaty to French Application Serial No. 1250162, filed Jan. 6, 2012, thedisclosure of each of which is hereby incorporated herein in itsentirety by this reference.

TECHNICAL FIELD

The invention relates to a method for fabricating a substrate, and to asemiconductor structure. The invention also relates to a method fortransferring a layer of a donor substrate onto a receiver substrate.

BACKGROUND

Thin layer transfers between semiconductor substrates using state of theart bonding approaches, such as the SMARTCUT® technology or othermolecular bonding techniques, require that the semiconductor wafers orsubstrates to be bonded both have a low surface roughness. For instance,EP 1 338 030 B1 discloses methods for forming a thin layer on a thicksupport, which can involve a first transfer step using the SMARTCUT®technology of a thin layer. The required surface quality for molecularbonding techniques is obtained by extensive polishing processes in orderto prepare the substrates before the bonding steps can be performed.

Therefore, there is a need for improved layer transfer techniques. Inparticular, there is a need for alternative bonding techniques in thesemiconductor industry.

This object is achieved with the inventive method for fabricating asubstrate, comprising the steps of providing a donor substrate with atleast one free surface, performing an ion implantation at apredetermined depth of the donor substrate to form an in-depthpredetermined splitting area inside the donor substrate, and providing alayer of an adhesive, in particular an adhesive paste, over the at leastone free surface of the donor substrate.

Surprisingly, this method has been found to yield satisfying resultswith semiconductor materials, as no polishing is required prior toapplying the adhesive layer. Thus, an ion layer can be implanted insidea donor substrate, and the substrate can be attached to or with theadhesive layer. The inventive method can, thus, advantageously, be usedfor mass-production of cheap semiconductor substrates and/orsemiconductor structures. Without being restricted to the following, thesubstrate can be a semiconductor, for example, a semiconductor wafer, asemiconductor substrate, a recycled semiconductor substrate, or even asemiconductor structure.

BRIEF SUMMARY

Depending on the conditions, if a layer of a natural oxide is formed onthe surface of the donor substrate or another kind of oxide layer isprovided on the surface of the donor substrate, a substrate-on-insulatorstructure can be formed using the inventive method. When the substrateis a semiconductor, a semiconductor-on-insulator can thus be fabricatedusing the inventive method.

In a variant of a preferred embodiment, the inventive method can furthercomprise the steps of providing a handle substrate with at least onefree surface, and attaching the donor substrate to the handle substratesuch that the layer of adhesive is provided between the at least onefree surface of the donor substrate and the at least one free surface ofthe handle substrate.

An advantage of the inventive method is that the main surfaces or mainsides by which the donor and handle substrates are attached to eachother by means of the adhesive layer do not require a polishing stepbefore the attachment step or any other surface preparation step toprovide a surface ready for molecular adhesion. Thus, the inventivemethod is particularly advantageous if used in the context oftransferring layers of semiconductor materials. Also, the adhesivematerial can be chosen to sustain very high temperatures (up to about1200° C.), which is not the case for typical prior art oxide bondinglayers.

Preferably, the adhesive can be a ceramic-based and/or a graphite-basedand/or a metal-based material.

Such adhesives are typically cheaper than polishing techniques followedby molecular bonding techniques known in the art but are still compliantwith a semiconductor fabrication environment. Some ceramic-based and/orgraphite-based and/or metal-based materials can be provided as one ortwo component systems, and can be mixed with water and/or special bindersystems in order to improve the properties of the adhesive. Furtherimproved results can be obtained when using ceramic-based and/orgraphite-based and/or metal-based materials in the context ofsemiconductor technologies.

Further preferred, the adhesive can be based on at least one of aluminumoxide, aluminum nitride, magnesium oxide, silicon dioxide, siliconcarbide, zirconium oxide, zirconium silicate, graphite, copper, andsilver.

Ceramic-based and/or graphite-based and/or metal-based materials offer awide range of physical and chemical properties. The selection of suchmaterials can be optimized depending on the required properties withrespect to temperatures, thermal conductivity, dielectric and mechanicalstrength.

Preferably, the adhesive can be provided over the entire at least onefree surface of the donor substrate and/or over the entire at least onefree surface of the handle substrate. In this way the adhesive with orwithout the handle substrate will provide sufficient stiffness over theentire substrate area.

In a further variant of a preferred embodiment, the inventive method canfurther comprise the step of providing a mold such that the adhesivelayer is provided with a predetermined geometry, in particular with ageometry matching that of the at least one free surface of the donorsubstrate and/or the at least one free surface of the handle substrate.

Using a mold is advantageous as the mold will constrain the geometry ofthe adhesive to a desired predetermined form. A variety of molds can befabricated, used and/or adapted to specific needs. For instance,silicone-based compounds or silicone rubber molds can be usedefficiently with a ceramic-based adhesive as they typically do not stickto the adhesive and can, therefore, easily be removed after thefabrication process.

Preferably, the inventive method can further comprise the step ofperforming at least one annealing at a temperature lower than atemperature required for a detachment at the level of the in-depthpredetermined splitting area.

Depending on the material chosen for the adhesive, at least oneannealing step can be required in order to densify the adhesive layer sothat it reaches it desired optimized properties with respect to thedonor and/or handle substrates. However, in order to prevent an unwantedinitialization of the layer detachment step prior to the densificationof the adhesive layer, the at least one annealing step is performed attemperatures lower than what is required for a detachment at the levelof the in-depth splitting area inside the substrate. Thus, such anannealing will have the advantage of outgassing water and/or organicsolvents of the adhesive paste while keeping the donor substrate withits in-depth weakened layer substantially intact. For instance, at leastone annealing is required when using one part or two part systems ofceramic-based compounds.

Preferably, the inventive method can further comprise the step ofremoving the mold after the annealing step.

When a mold is used, the mold can advantageously be kept during theannealing step until the adhesive layer reaches an appropriate density.The mold may, however, be removed before or after detaching a layer ofthe donor substrate, with the advantage of being reusable in asubsequent process. For instance, silicone rubber molds can be recycledand/or reused advantageously when used with a ceramic-based adhesive asthey typically do not stick to such adhesives.

Preferably, the inventive method can further comprise the step ofdetaching a remainder of the donor substrate at the level of thein-depth predetermined splitting area.

Thus, a layer of the initial donor substrate can be transferred onto theadhesive layer. Depending on the preferred embodiment, a layer of thedonor substrate can be transferred to form a semiconductor structure,according to the invention.

Preferably, the inventive method can further comprise the step ofreusing the remainder of the donor substrate as a new donor substrate.

Since the inventive method does not require polishing of the surfaceover which the adhesive layer will be attached, the remainder of thedonor substrate could immediately be reused or recycled as a new donorsubstrate in a new subsequent layer transfer process, thereby reducingthe number of process steps.

Advantageously, the adhesive layer can have a thickness of at least 0.1μm.

In the case of transferring a thin layer of a semiconductor material,not thick enough to ensure its mechanical stability alone, the adhesivelayer can provide the necessary thickness to ensure the mechanicalstability of the transferred thin layer. When no handle substrate isused, the mechanical stability of a transferred thin layer can beprovided with an adhesive layer having a thickness of about 20 μm to 1mm. However, when the adhesive is used as bonding layer between thedonor substrate and the handle substrate, a thickness of about 0.1 μm to10 μm can be sufficient.

The object is also achieved with the inventive semiconductor structurecomprising a semiconductor layer, and a layer of a ceramic-based and/ora graphite-based and/or a metal-based adhesive provided over one mainside of the semiconductor layer.

The invention can be optimized for semiconductor applications. Thus, asemiconductor structure, according to the invention, can be produced fora low cost and can be used for a variety of semiconductor applicationswithout limiting the quality of the semiconductor layer of interest.Ceramic-based and/or graphite-based and/or metal-based adhesives havethe advantage of being compliant with a semiconductor fabricationenvironment.

In a variant of a preferred embodiment, the inventive semiconductorstructure can further comprise an insulating layer between thesemiconductor layer and the adhesive layer. Thus, the invention alsoprovides cheap semiconductor-on-insulator structures for use in severalsemiconductor applications.

In a further variant of a preferred embodiment, the inventivesemiconductor structure can further comprise a substrate attached to theadhesive by an attachment surface such that the adhesive is providedbetween the main side of the semiconductor layer and the attachmentsurface of the substrate. A further advantage of the invention is thusthe cheap production of semiconductor structures comprising also atleast one other substrate and which can be applied to various fieldsinvolving semiconductor technologies.

Advantageously, the adhesive of the inventive structure can comprise atleast one of an aluminum oxide, an aluminum nitride, a magnesium oxide,a silicon dioxide, a silicon carbide, a zirconium oxide, a zirconiumsilicate, graphite, copper, and silver. Thus, the inventivesemiconductor structure can be specifically adapted to a particular usedepending on its properties relative to temperature, thermalconductivity, dielectric and mechanical strength.

Preferably, the adhesive of the inventive semiconductor structure can beprovided on the entire surface of the main side of the semiconductorlayer and/or on the entire attachment surface of the substrate. Thus, aninventive structure, with or without a substrate, is provided withsufficient stiffness over the entire semiconductor layer area.

Further preferred, the adhesive layer of the inventive semiconductorstructure can have a thickness of at least 20 μm and up to 1 mm when nosubstrate is used.

Alternatively, the adhesive layer of the inventive semiconductorstructure can have a thickness of at least 0.1 μm and up to 10 μm whenthe adhesive layer is provided between the main side of thesemiconductor layer and the attachment surface of the substrate.

Thus, the inventive semiconductor structure has an advantage in that thesemiconductor layer is provided with sufficient mechanical stability,regardless of its thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention can be combined in orderto obtain further embodiments. The invention will be described in moredetail hereafter, based on advantageous embodiments described incombination with the following figures:

FIG. 1A schematically illustrates a first embodiment of the inventivemethod;

FIG. 1B schematically illustrates a variant of the product of the firstembodiment;

FIG. 2 schematically illustrates a second embodiment of the inventivemethod;

FIG. 3A schematically illustrates a third embodiment of the inventivemethod;

FIG. 3B schematically illustrates a variant of the product of the thirdembodiment.

In the various embodiments described hereafter and in FIGS. 1A-3B, thesame reference numbers have been used for identical elements or elementssharing similar roles.

DETAILED DESCRIPTION

FIG. 1A schematically illustrates a first embodiment of the inventivemethod. According to the inventive method, in step S101 of FIG. 1A, afirst substrate 1 is provided. In the first embodiment, the substrate 1is a silicon wafer. However, the inventive method is not restricted tosilicon or silicon-based substrates and can be applied to othermaterials, in particular to any other semiconductor, for instance, SiCor GaN, or to any other semiconductor-based material.

Following the inventive method, step S102 of FIG. 1A illustrates an ionimplantation step to form a predetermined splitting area 2 inside thesubstrate 1, at a predetermined depth d with respect to one of the mainsurfaces 7 of the substrate 1. The predetermined depth d is typically ina range of about 1 nm up to several hundreds of μm, depending on theapplication. According to one practical example of the first embodimentof the inventive method, H and/or He ions are implanted into the siliconwafer 1. However, in other embodiments, other ions may be used.

According to the inventive method, step S102 in FIG. 1A is followed bystep S103 of providing a layer of an adhesive 4 over the main surface 7of the substrate 1, thereby forming an intermediate semiconductorstructure 9.

According to a preferred variant of the inventive method, the adhesiveis a ceramic-based material. In further embodiments of the inventivemethod, the adhesive could, however, also be a graphite-based and/or ametal-based material comprising one of graphite, copper, and silver. Theceramic-based adhesive paste can be based on aluminum oxide, and/or analuminum nitride, and/or a magnesium oxide, and/or a silicon dioxide,and/or a silicon carbide, and/or a zirconium oxide, and/or a zirconiumsilicate, depending on the desired final application, in particulardepending on the physical and/or chemical properties of the chosenmaterial for the substrate 1. For instance, CERAMCAST™ materials fromAremco Products, Inc. can be adapted for various embodiments of theinventive method. Such ceramic-based materials can be provided either inone or two component systems. One component systems can be mixed withwater or other systems to improve moisture resistance and can be set atroom temperature in several hours, and then baked and/or annealed attemperatures of about 90° C. to about 150° C. in a few hours to provideoptimal electrical and mechanical properties. Two component systems canbe mixed with water and/or other components and have varying set timesand can be annealed at similar temperatures but often in shorter times.Depending on the component, some final annealing phases can reachtemperatures of about 250° C., but have the advantage of being fasterthan at lower temperatures.

According to a preferred variant, the adhesive layer 4 is deposited witha thickness of at least 0.1 μm, preferably from 20 μm to 1 mm. Once, theadhesive has hardened, the thickness of the adhesive layer 4 providesfor the necessary stiffness required for the mechanical stability of thefinal structure, as will be described in the subsequent steps of thefirst embodiment.

Step S103 is followed by one or more annealing steps. In a practicalexample, the annealing is performed for about 2 hours at a temperatureof about 100° C., which, according to the invention, is selected lowerthan the temperature which is necessary for detaching a layer of thesubstrate 1 at the level of the in-depth predetermined splitting area 2of implanted ions. According to a variant of the inventive method, theannealing step can be followed by a second annealing step of about 2hours at a temperature of about 150° C., also lower than the temperatureof layer splitting at the level of the in-depth predetermined splittingarea 2. In further variants, depending on the choice of adhesivematerial, at least one annealing step could be performed at roomtemperature. The at least one annealing has the advantageous effect ofoutgassing water and/or organic solvents of the ceramic adhesive layer 4without, however, substantially weakening the in-depth predeterminedsplitting area 2 inside the substrate 1.

In step S104, as illustrated in FIGS. 1A, 1B, a final annealing isperformed at a higher temperature in order to detach a layer of thesubstrate 1 at the level of the predetermined splitting area 2.Typically, the final annealing is performed for about 2 hours at atemperature of about 400° C. The final annealing temperature and itsduration can, however, be adapted as a function of the implanted ionsand the substrate properties.

As a result of the method, according to the first embodiment, a layer 1′of the substrate 1 remains attached to the adhesive layer 4, forming aninventive semiconductor structure 91. The layer 1′ has a thickness ofabout 1 nm up to several hundreds of μm, corresponding to the depth d ofthe in-depth predetermined splitting area 2 formed by the implanted ionsbelow the main surface 7 of the initial donor substrate 1. The adhesivelayer 4 provides for the stiffness of the semiconductor structure 91, inparticular for the mechanical stability of the transferred layer 1′.

In another variant, the remainder 1″ of the initial substrate 1 can bereused or recycled as a new donor substrate in a new step S101 of asubsequent layer transfer process, according to the inventive method. Asno particular surface quality is needed for the attachment surface 7,there is no obligation to polish the surface 7″ of the remainder 1″prior to reuse.

Contrary to layer bonding or layer transfer techniques by molecularbonding, which require bonding surfaces with a very low roughness, and,thus, polishing steps prior to the layer bonding or layer transfersteps, according to the inventive method, the surface quality of a mainsurface 7 of the substrate 1 does not have to be as good as to enablebonding via molecular forces. Thus, an advantage of the inventive methodis that a main surface 7 of the substrate 1 does not require a priorpolishing step or any other surface preparation step to provide asurface ready for molecular adhesion. A further advantage is that anadhesive material is simple to provide in the required thickness, inparticular in order to obtain a desired mechanical stability of theintermediate structure and/or the inventive final structure.

Depending on the desired final structure, a layer 11 of a natural oxideor a deposited dielectric, such as SiO₂ or the like, may be present onor over at least the main surface 7 of the substrate 1 prior to the stepof providing the layer of adhesive 4. Thus, depending on the chosenmaterial for the adhesive layer 4 and the substrate 1 and theexperimental conditions, the product of the inventive method can be asemiconductor-on-insulator (SOI) type structure. For instance, in thevariant of the final product of the first embodiment as illustrated inFIG. 1B, a silicon-on-insulator structure 91′ is obtained, wherein alayer of a natural oxide 11 forms an insulating layer between thesemiconductor layer 1′ and the adhesive layer 4.

A second embodiment of the invention is illustrated in FIG. 2. StepsS201 and S202 of the second embodiment represented in FIG. 2 correspondto steps S101 and S102 of the first and second embodiments. It is,therefore, referred back to the description above. Like in the first andsecond embodiments, the materials used for the donor substrate 1 and theimplanted ions used for the in-depth predetermined splitting area 2 areonly used for illustrative purposes, and other materials and/or ions canbe used in further embodiments.

According to an advantageous variant of the inventive method, the secondembodiment further comprises step S203 illustrated in FIG. 2 ofproviding a mold 3. When using ceramic adhesive materials, like, forinstance, in the first embodiment, silicone molding compounds, like,e.g., EZ-CAST™ silicone rubber molds, can be adapted to variousembodiments of the present invention.

Such a mold 3 made out of a silicone rubber compound has the advantagethat the deposited adhesive layer 4 that will be used later in theprocess does not stick to it. The mold 3 is cast from a master form suchthat it comprises a pattern 6 with a geometry matching the geometry ofthe substrate 1. In the second embodiment, since the substrate 1 is asilicon wafer, the pattern 6 of the mold 3 thus matches the geometry ofsuch a wafer, in particular, the geometry of the main bottom surface ofthe pattern 6 in the mold 3 matches the geometry of the at least onefree surface 7 of the substrate 1, and the depth of the side walls 10 ofthe pattern 6 can be adjusted to a desired value depending on thedesired thickness of the adhesive layer 4.

As illustrated by step S204 in FIG. 2, in the second embodiment, anadhesive such as the adhesive used for the first embodiment is appliedon the entire main surface of the pattern 6 of the mold 3, and fills atleast partially a predetermined thickness of the pattern 6. Thus, anadhesive layer 4 is formed with a geometry based on the geometry of thepattern 6, in particular with a geometry matching the main surface 7 ofthe silicon wafer or substrate 1. In the second embodiment, the adhesivelayer 4 has a thickness of at least 0.1 μm, preferably from 20 μm to 1mm, which is inferior or equal to the depth of the side walls 10 of thepattern 6 inside the mold 3.

As can be seen on FIG. 2, step S204 of the second embodiment is followedby step S205 of placing the substrate 1 with the in-depth predeterminedsplitting area 2 of implanted ions on or in the pattern 6 of the mold 3such that the entire surface 7 of the substrate 1 is in contact with theentire surface of the adhesive layer 4 present inside the pattern 6.

In the second embodiment, and as illustrated in step S205, the adhesivelayer 4 covers the main surface of the pattern 6, but does notnecessarily fill up to the thickness of the side walls 10 of the pattern6. A part of the side walls 10 of the pattern 6 can thus partiallyoverlap with the side walls of the substrate 1 so that an alignmentbetween the shape of the substrate 1 and the adhesive 4 is achieved.

According to an advantageous variant of the inventive method, the secondembodiment further comprises a step S206 after step S205, as illustratedin FIG. 2. Step S206 comprises a first annealing phase followed by afinal annealing or detachment step, which is similar to step S104 of thesecond embodiment, and presents similar functions and advantages. It is,therefore, referred back to the description above.

Step S207 of the second embodiment consists in removing the mold 3 suchthat the inventive structure 92 is obtained. The mold 3 can either bediscarded or recycled or reused in step S203. Thus, in the secondembodiment, a layer 1′ of the initial semiconductor substrate 1 istransferred onto the ceramic-based adhesive layer 4, and a remainder 1″of the initial substrate 1 is detached during the detachment step inorder to obtain final semiconductor structure 92.

In further embodiments, the remainder 1″ of the initial donor substrate1 can be recycled, without necessarily going through a polishing step,as a new donor substrate in step S201. In further embodiments, aninsulating layer may have formed naturally or have been deposited atleast on the surface 7 of the substrate 1, such that asemiconductor-on-insulator layer is transferred onto the adhesive layer4, similar to what is illustrated in FIG. 1B for the first embodiment.

In the second embodiment, using a mold 3 has the advantage that thegeometry of the deposited adhesive layer 4 is constrained and providesthe final structure 92 with sufficient stiffness and mechanicalstability over the entire surface 7 of the transferred layer 1′.Furthermore, the mold 3 of the second embodiment has the advantage thatthe adhesive layer 4 does not stick to it so that the step of removingthe mold 3 does not risk damaging the final structure 92. A furtheradvantage is that such a mold 3 can be reused or recycled, which cansave further productions costs.

FIG. 3A illustrates the inventive method in a third embodiment,comprising steps S301 and S302 of providing a donor substrate 1 andimplanting ions inside the substrate 1 such as to form an in-depthpredetermined splitting area 2. Steps S301 and S302 of the thirdembodiment are similar to steps S101 and S102 of the first embodimentand to steps S201 and S202 of the second embodiment. It is, therefore,referred back to the description above. Like in the previousembodiments, the materials used for the donor substrate 1 and theimplanted ions used for the in-depth predetermined splitting area 2 areonly used for illustrative purposes, and other materials and/or ions canbe used in further embodiments.

As illustrated in FIG. 3A, the third embodiment further comprises stepS303 of providing a second type of mold 3′. This step is similar to stepS203 of the second embodiment. It is, therefore, referred back to thedescription above. However, the pattern 6′ of the mold 3′ in the thirdembodiment is different from the pattern 6 of the mold 3 in the secondembodiment, as in the third embodiment the pattern 6′ extends throughthe entire thickness of the mold 3′, forming a hole with side walls 10′in the mold 3′, the main section of pattern 6′ matching the surface 7 ofthe donor substrate 1, for instance, the silicon wafer of the firstembodiment.

The third embodiment further comprises step S304. In this step, thedonor substrate 1 is placed at least partially in the pattern 6′ of themold 3′, overlapping with the side walls 10′ and leaving at least apredetermined thickness of the pattern 6′ for applying an adhesive layer4. The properties of the adhesive layer 4 are similar to those describedin the previous embodiments. It is, therefore, referred back to thedescription above for details. In the third embodiment, the adhesivelayer 4 covers the entire surface 7 of the substrate 1 and has athickness of at least 0.1 μm, preferably from 0.1 μm to 10 μm, whichdoes not fill the remaining portion of the pattern 6′. In otherembodiments of the inventive method, the thickness of the adhesive layer4 can be more important, it can even be at least equal to the depth ofthe side walls 10′. In further embodiments, the mold 3′ can be placedover the substrate 1 such that no side wall 10′ of the pattern 6′overlaps with the substrate 1. All these embodiments can be combined toproduce even further embodiments.

Then, step S305 consists in providing a second substrate 5. Asillustrated in step S305 of FIG. 3A, the second substrate 5 is attachedby one of its free surfaces, in particular an attachment surface 8, tothe adhesive layer 4. In the third embodiment, the geometry of thesecond substrate 5 is such that its attachment surface 8 matches thegeometry of the main surface 7 of the first substrate 1, and thethickness of the ceramic-based adhesive layer 4 is such that the secondsubstrate 5 also overlaps partially with the side walls 10′ inside thepattern 6′. Depending on the embodiment, other situations may requirethat the geometry of the attachment surface 8 of the second substrate 5is different than that of the main surface 7 of the first substrate 1,and/or that the substrate 5 is placed on the mold 3′, in particular overthe pattern 6′, but does not overlap with the side walls 10′ of thepattern 6′. In the third embodiment, the second substrate 5 can beanother semiconductor wafer, for example, comprising silicon and/orquartz and/or any other semiconductor, or a plastic-based material orthe like, or a ceramic-based material like aluminum nitride or the like,or a metal like molybdenum or the like.

As a variant, the second substrate 5 could be first placed in the mold3′ at step S304 such that the adhesive 4 would be applied to theattachment surface 8 of the second substrate 5 before attachingsubstrate 1 by its main surface 7 to the adhesive layer 4 in step S305.

In step S306, according to further preferred variants of the inventivemethod, the third embodiment further comprises performing annealing anddetachment steps, which are similar to step S104 of the first embodimentand step S206 of the second embodiment, and present similar functionsand advantages. It is, therefore, referred back to the descriptionabove.

After the detachment step in S306, the remainder 1″ of the initial donorsubstrate 1, for instance, the silicon wafer of the first embodiment, isremoved and can be reused in step S301 or recycled. Like mentionedabove, for the reuse no additional polishing step is necessary.

After step S306, like step S207 of the second embodiment, step S307 ofthe third embodiment, as illustrated in FIG. 3A, also comprises a stepof removing the mold 3′, which can then be recycled, for instance, inStep S303 or discarded. In step S307 of the third embodiment, asillustrated in FIG. 3A, a final semiconductor structure 93, according tothe invention, is obtained. The semiconductor structure 93 comprises athin silicon layer 1′ attached to a handle substrate 5 via theceramic-based adhesive layer 4. In the third embodiment, according toone aspect of the invention, the geometry of the adhesive layer 4matches the geometry of the thin layer 1′ and/or that of the handlesubstrate 5. In particular, the adhesive layer 4 is deposited over theentire respective attachment surfaces 7, 8 of the thin layer 1′ and thesecond substrate 5.

Similar to the first embodiment, depending on the experimentalconditions and the user requirements, a natural oxide may have formed ora further insulating layer 11 may have been provided over the surface 7of the initial donor substrate 1 such that a final structure 93′, asillustrated in FIG. 3B, is obtained. The product of the inventivemethod, according to the third embodiment and its variants, can thus bea semiconductor-on-insulator (SOI) structure comprising an insulatinglayer 11 provided between a thin layer 1′ of a semiconductor substrate 1and a ceramic-based adhesive layer 4 to which a second substrate 5 isattached by an attachment surface 8.

In a preferred variant of an embodiment of the invention, the adhesive,in particular the ceramic-based compound used for the adhesive layer 4,can be chosen depending on its constant of thermal expansion (CTE). Forinstance, if a further application of the final substrate or thesemiconductor structure (91, 91′, 92, 93, 93′), according to theinvention, requires epitaxial deposition on the thin layer 1′, arequirement for selecting the proper adhesive, in particular the properceramic-based adhesive, can be that its CTE should be compatible withthe CTE of the epitaxial material.

Thus, the inventive method provides with an alternative technique forattaching a layer of a donor substrate 1, for example, to a handlesubstrate 5. An advantage of the inventive method is that the attachmentsurfaces 7, 8 of the substrates 1, 5 to be attached do not requirepolishing steps prior to the attachment step. Preferred variants of theinventive method use ceramic-based adhesives that can advantageously beselected in order to be compliant with the thermal and/or mechanicaland/or conductive properties of the substrate 1, and that offer thenecessary stiffness for the mechanical stability of the transferredlayer 1′ of the donor substrate 1. Further preferred variants of theinventive method have the advantage of using a mold 3, 3′ which,depending on the chosen material, can be reused, in particular recycledfor further embodiments of the inventive method. Further advantageousembodiments of the invention allow recycling a remainder 1″ of the donorsubstrate 1. The invention further provides a diversity of inventivestructures, for instance, semiconductor-on-insulator structures, whichhave advantageous applications in various technological environments.

The embodiments described above and further variants of the sameembodiments can be combined in order to realize even further embodimentsof the invention.

The invention claimed is:
 1. A method for fabricating a substrate,comprising the steps of: providing a donor substrate with at least onefree surface and at least one sidewall; performing an ion implantationat a predetermined depth within the donor substrate to form apredetermined splitting area inside the donor substrate at thepredetermined depth; positioning the donor substrate at least partiallywithin a recess or hole in a mold, the mold including at least onesidewall within the recess or hole having a geometry matching a geometryof the at least one sidewall of the donor substrate; and providing alayer of an adhesive paste over an entirety of the at least one freesurface of the donor substrate while the at least one sidewall of themold within the recess or hole at least partially overlaps the at leastone sidewall of the donor substrate, the at least one sidewall of themold confining the layer of the adhesive paste to a region adjacent theat least one free surface of the donor substrate.
 2. The method of claim1, further comprising the steps of providing a handle substrate with atleast one free surface, and attaching the donor substrate to the handlesubstrate such that the layer of adhesive paste is provided between theat least one free surface of the donor substrate and the at least onefree surface of the handle substrate.
 3. The method of claim 1, whereinthe adhesive paste is at least one of a ceramic-based, a graphite-based,and a metal-based material.
 4. The method of claim 3, wherein theadhesive paste is based on at least one of aluminum oxide, aluminumnitride, magnesium oxide, silicon dioxide, silicon carbide, zirconiumoxide, zirconium silicate, graphite, copper, and silver.
 5. The methodof claim 2, wherein the layer of adhesive paste is applied to at leastone of the entire at least one free surface of the donor substrate andthe entire at least one free surface of the handle substrate prior toattaching the donor substrate to the handle substrate such that thelayer of adhesive paste is provided between the at least one freesurface of the donor substrate and the at least one free surface of thehandle substrate.
 6. The method of claim 1, further comprising annealingthe layer of adhesive paste at an annealing temperature lower than atemperature required for a detachment at the level of the predeterminedsplitting area.
 7. The method of claim 1, further comprising the step ofdetaching a remainder of the donor substrate at the level of thepredetermined splitting area.
 8. The method of claim 7, furthercomprising the step of reusing the remainder of the donor substrate as anew donor substrate.
 9. The method of claim 1, wherein the layer ofadhesive paste has a thickness of at least 0.1 μm.
 10. Asemiconductor-on-insulator (SOI) substrate, comprising: a semiconductorlayer; and a layer of hardened adhesive paste provided over an entiresurface of one main side of the semiconductor layer, the layer ofhardened adhesive paste having a thickness in a range extending from 20μm to 1 mm so as to provide stiffness required for the mechanicalstability of the SOI substrate, the hardened adhesive paste at least oneof a ceramic-based adhesive, a graphite-based adhesive, and ametal-based adhesive.
 11. The SOI substrate of claim 10, furthercomprising an insulating layer between the semiconductor layer and thelayer of hardened adhesive paste.
 12. The SOI substrate of claim 10,further comprising a substrate having an attachment surface in physicalcontact with the hardened adhesive paste such that the hardened adhesivepaste is provided between the main side of the semiconductor layer andthe attachment surface of the substrate.
 13. The SOI substrate of claim10, wherein the hardened adhesive paste comprises at least one of analuminum oxide, an aluminum nitride, a magnesium oxide, a silicondioxide, a silicon carbide, a zirconium oxide, a zirconium silicate,graphite, copper, and silver.
 14. The SOI substrate of claim 12, whereinthe hardened adhesive paste provided on at least one of the entiresurface of the main side of the semiconductor layer and the entireattachment surface of the substrate.
 15. The method of claim 2, whereinthe adhesive paste is at least one of a ceramic-based, a graphite-based,and a metal-based material.
 16. The method of claim 1, whereinpositioning the donor substrate at least partially within the recess orhole in the mold comprises positioning the donor substrate at leastpartially within a recess or hole in a silicone rubber mold.
 17. Themethod of claim 16, wherein positioning the donor substrate at leastpartially within a recess or hole in the silicone rubber mold comprisespositioning the donor substrate at least partially within a recess inthe silicone rubber mold.
 18. The method of claim 9, wherein the layerof adhesive paste has a thickness in a range extending from 20 μm to 1mm.